Perform a bounds check to fix a random Termcurses crash that I haven't been able to track down. The result of of the subtraction for priv->keycount should never be negative, but I think sometimes it is, so better to catch it and avoid a crash until I can track it down.

system/termcurses/Kconfig

@@ -12,6 +12,8 @@ config SYSTEM_TERMCURSES
 		Terminal emulation library for curses support on TTY type consoles
 		such as serial port, CDCACM, telnet, etc.
 
+		Must deselect CONFIG_DISABLE_POLL!
+
 config SYSTEM_TERMCURSES_VT100
 	bool "Terminal pdcurses support for VT-100"
 	depends on SYSTEM_TERMCURSES

system/termcurses/tcurses_vt100.c

@@ -1041,6 +1041,13 @@ static int tcurses_vt100_getkeycode(FAR struct termcurses_s *dev, FAR int *speci
       /* Update keycount and keybuf */
 
       priv->keycount -= x;
+      if (priv->keycount < 0)
+        {
+          /* Hmm, some bug.  Better to simply ignore than to crash */
+
+          priv->keycount = 0;
+        }
+
       if (priv->keycount != 0)
         {
           memmove(priv->keybuf, &priv->keybuf[x], priv->keycount);